The present invention relates to improvement in or relating to a band-restricted, quaternary-level waveform generator for deriving a band-limited, shaped waveform from a sequence of quaternary-level square waves with a fixed pitch between any two adjacent levels out of four, which is used in a 16 quardrature amplitude modulation (QAM),four-level frequency modulation (FM), or similar digital modulation schemes.
For deriving a shaped waveform of four levels from a quaternary-level square wave sequence by subjecting it to a predetermined band restriction, there have heretofore been used an analog or digital filtering method which shapes the quaternary-level square wave signal by a band restriction analog or digital low-pass filter, and a waveform table ROM method according to which, with respect to all combinations of k symbols (k being a constraint length due to intersymbol interference caused by predetermined band restriction), the waveform of the k-th symbol of the above-mentioned shaped waveform after the band restriction is computed on off-line basis and prestored in a ROM (Read Only Memory) as a waveform table and is then read out therefrom and subjected to a D/A conversion
With the analog filtering method, however, the low-pass filter for band restriction use is usually high-order, and hence is large in the scale of its circuit structure, and when the low-pass filter is formed by an analog element, it calls for compensation for an environmental change and timing aging and fine adjustment and is not suitable for fabrication as an IC. Thus, the analog low-pass filter has difficulties in miniaturization, economization and stabilization.
If the low-pass filter is implemented by a digital signal processing device through utilization of the A/D and D/A conversion techniques, its stabilization can be achieved but its circuit scale and power consumption will be large.
On the other hand, the digital processing method utilizing the waveform table ROM is generally excellent in terms of miniaturization, economization and stabilization, but letting the number of samples of the D/A conversion per symbol be represented by S, the capacity of the ROM (the number of memorized words for D/A conversion) is S.multidot.4.sup.k words. This value is 2.sup.k times larger than the S.multidot.2.sup.k -word capacity which is needed in a modulation scheme, such as QPSK (quadrature phase shift keying) using a shaped waveform of a binary-level square wave sequence is generated from the waveform table ROM with the same sample number S. Since the capacity becomes enormous with an increase in k, this method is impractical when k becomes comparably large.